The speaker is describing the moment someone walks out and shuts the door, and how much weaker it leaves them feeling. The phrase "much more fragile" comes back again and again, almost like a heartbeat, as if the speaker can't stop circling the same hurt. "Than before" suggests they were already delicate, and the departure only made it worse.
The garden is the poem's main image. It's "huge," which makes the speaker seem small, and being "all alone and afraid" in it turns a space that could be peaceful into something exposed and lonely. The line "Like you always do" is quietly important: it hints that this isn't the first time, and that leaving is a pattern. The mood is sad, anxious, and a little resigned. Nothing dramatic happens, no shouting or argument, just a closed door and the silence afterward. That plainness makes the feeling seem honest, like someone saying out loud what they can't take back.